A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The transfer of the pattern on to the substrate is carried out by a projection system. Software is used in the control of the projection system to ensure that the pattern is aligned correctly with the substrate and with patterns already transferred onto the substrate, as well as ensuring that the pattern has the correct shape and orientation. Specifically, the software uses measurements taken by the lithographic apparatus to calculate optimal adjustments for the lithographic apparatus during exposure; and transmits those adjustments to the parts of the lithographic apparatus that make the adjustments. The adjustments may be the settings of a lens, or the XY- and Z-positions or tilt of the substrate, substrate holder or substrate table. There is also a calibration state associated with the software to ensure that the software is calibrated with the projection system and the rest of the lithographic apparatus(es) to which the software is associated.
This software is frequently upgraded or changed. A change in the software may be in order to create a different pattern on the substrate, or to fix bugs in old software; an upgrade of the software may be to create a more efficient pattern transfer process, for example. Often, the bug fixes involve a slight change in XY positioning (i.e. in the plane of the substrate) or in Z-direction height or tilt of the substrate (usually because the bug itself was a misalignment in positioning as a result of any parts of the lithographic apparatus such as an alignment laser, etc.) The person skilled in the art will be aware of a multitude of reasons why the software controlling the projection system may be upgraded or changed.
It is desirable that software upgrades or changes be timed so that software upgrades do not occur in between the radiation of different layers on the same substrate; or even different substrates in the same “lot”. In practice, this may be very difficult. This is because new software-functionality is likely to make the projection of a pattern onto a substrate layer different from the old software-functionality that was used for previous layers and the substrate or lot in question may effectively be ruined as the layers are likely to have overlay errors as a result of layers of one type being deposited on layers of a different type. Furthermore, it may be difficult to time the implementation of a new software functionality with the beginning of a lot of substrates on a series of machines, particularly as the machines are often at different points in a lot at any one time. This difficulty is known as a work-in-progress impact. Furthermore, old software-functionality and new software-functionality may have been calibrated slightly differently, which causes the same work-in-progress impact.
Because of this reduction in quality of-lots of substrates following a software upgrade, users of lithographic apparatus are reluctant to upgrade their software, potentially effecting their market positions.